Voltage-dividing DC circuit breaker and method

ABSTRACT

The voltage dividing DC circuit breaker comprises serially connected high voltage and low voltage switches to carry load current. Serial capacitors are connected in parallel thereto and a crossed field switching device is connected in parallel to the low voltage switch. When the low voltage and high voltage switches are opened, current is forced through the crossed field switching device. When the crossed field switching device is turned off, the arcing high voltage switch is forced below its chopping current. The open circuit voltage is divided across the capacitors.

BACKGROUND

1. Field of the Invention

This invention is directed to a voltage dividing DC circuit breaker, andthe method of operating the breaker.

2. The Prior Art

The prior art includes patents such as K. T. Lian and W. F. Long U.S.Pat. No. 3,641,358, which uses an offswitching device which requires thefull voltage holdoff capability corresponding to the open-circuitvoltage plus over-voltages. This is also true of M. A. Lutz and W. F.Long, U.S. Pat. No. 3,660,723, as well as K. T. Lian re-issue U.S. Pat.No. Re27,557 (re-issue of U.S. Pat. No. 3,534,226). Thus, the priorstructure require a full voltage circuit interrupter to interrupt theline voltage. It has been conceived that a plurality of suchinterrupters could be connected in series to achieve the requiredvoltage rating. However, a series of such devices can raise otherproblems. For example, when turning off a plurality of seriallyconnected devices, they must be turned off in appropriate relationshipto each other, an unequal voltage division results which can causedestruction. Even onswitching of such serially connected devices can bedifficult if the devices in the circuit are not appropriately designed.

SUMMARY

In order to aid in the understanding of this invention, it can be statedin essentially summary form that it is directed to a voltage dividing DCcircuit breaker and method. The DC circuit breaker includes seriallyconnected high and low voltage switches which carry the load current.Serially connected first and second capacitors are connected in parallelto the switches. A crossed field switching device is connected inparallel to the low voltage switch and second capacitor. Thus, when thelow voltage switch is opened, the current is forced through the crossedfield switching device. The high voltage switch is opened and conductscurrent by arcing. When the crossed field switching device isoffswitched, current in the open and arcing high voltage switch isdriven below its chopping value and voltage across the open switchesrises in accordance with the series capacitor values. The methodcomprises operating the voltage dividing DC circuit breaker in thatmanner.

Accordingly, it is an object of this invention to provide a voltagedividing DC circuit breaker and method, such that voltage is dividedacross switches therein when the circuit breaker is made non-conducting.It is another object to provide a voltage dividing DC circuit breakerwherein low voltage components can be employed to perform the majorswitching functions. It is another object to divide voltage acrossserially connected switches in accordance with capacitive division.

Other objects and advantages of this invention will become apparent froma study of the following portion of the specification, and the claimsand the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic electrical circuit diagram of the voltage dividingDC circuit breaker in accordance with this invention.

FIG. 2 is a graph of voltage versus time showing the voltage rise acrossthe high voltage switch in the series-connected switches of thisbreaker.

FIG. 3 is a graph showing the voltage versus time across the crossedfield offswitching device in the circuit breaker of this invention.

FIG. 4 is a schematic electric circuit diagram of the voltage dividingDC circuit breaker of this invention showing it as incorporatingmultiple stages for successive impedance increase.

DESCRIPTION

FIG. 1 illustrates the voltage dividing DC circuit breaker 10 of thisinvention at the right-hand end of the schematic circuitry of FIG. 1.The left end of the schematic circuitry illustrates a power supply 12and an electric load represented by resistance 14 and inductance 16.Power supply 12 is serially connected with the load through circuitbreaker 10, so that when circuit breaker is opened current flow throughthe load is cut off. A generation and distribution circuit, including aload, is shown in more detail in M. A. Lutz and W. F. Long U.S. Pat. No.3,660,723, and the power supply, distribution and load are representedin more detail in that patent. Power supply 12 is a high voltage powersupply, with voltage levels such that it is difficult to offswitchcurrent against such a high voltage with present-day commercialequipment. In terms of equipment presently available, in experimentalusage, circuit breaker 10 is capable of offswitching in circuits whichhave a supply voltage above 100 kv. The same equipment is capable ofhandling current of 1000 amps or higher.

Buses 18 and 20 form the two terminals of circuit breaker 10. Switch 22is connected to bus 18 and to an intermediate bus at junction 24. Switch26 is connected between junction 24 and bus 20. When both switches 22and 26 are closed, power supply 12 supplies current through the load,through the closed switches and through the buses. When it is desiredthat load current be offswitched, these switches are opened.

Switch 22 is a switch which is capable of continuously carrying the linecurrent, such as a commercially available interrupter. A suitable vacuuminterrupter is shown in T. H. Lee et al patent 3,411,038. The Leestructure is preferably furnished with electrodes of refractory metal,to provide a high chopping current value. Switch 26 is a switch which iscapable of carrying full load current and is capable of opening andproducing a sufficiently high arc voltage to drive the circuit currentinto the cross field switch device 28 which is connected in parallelthereto. Thus, switch 26 can be a standard SF₆ filled circuit breaker orcan be a switch as described in Noel E. Reed patent application Ser. No.255,665.

Crossed field switch device 28 is connected between junction 24 and bus20. It is a device which is capable of carrying the full load current,but need only carry the current for a short time. Furthermore, while itmust offswitch against the load current, it need not carry the full opencircuit voltage of power supply 12 or the overvoltage offswitchingtransients. Suitable offswitching devices are shown in K. T. Lian U.S.Pat. No. 3,534,226, and R. C. Knechtli U.S. Pat. No. 3,638,061 and G. A.G. Hofmann and R. C. Knecktli U.S. Pat. No. 3,558,960. Other relatedcrossed field offswitching devices are also useful in this service.

Capacitors 30 and 32 are serially connected between buses 18 and 20, andthe junction 34 therebetween is connected through the intermediate busto the junction 24. Capacitor 36 is connected across buses 18 and 20.

The method of operation of the voltage dividing DC circuit breaker 10 isas follows. Assume that power supply 12 is supplying 1000 amps DC, andthe load has a voltage drop of 100 kv. In normal operation the switches22 and 26 are closed so that all of the load current passes therethroughand there is not substantial voltage drop thereacross. When it becomesdesirable to offswitch the current through the load, switch 26 isopened. The voltage drop caused by arcing of open switch 26 causestransfer of current through crossed field switch 28, at time t_(o) inFIGS. 2 and 3. This permits the deionization of open switch 26, with thevoltage drop represented in FIG. 3 being the normal conducting voltagedrop of crossed field switch device 28. The voltage drop in such a caseis on the order of 500 v. Switch 22 is opened at time t₁, preferably atthe same time as switch 26 is opened, and the arc voltage dropthereacross is represented from t₁ to t₃ in FIG. 2. To decrease thetotal offswitching interval, both switches 22 and 26 can be opened atthe same time. Next, crossed field switch 28 starts to offswitch at timet₂ with reduction in current flow through the series connection ofswitches 22 and 28. Previously, switch 26 is opened and deionized. Asthe voltage drop rises across crossed field switch 28 as it ceasesconduction, the current therethrough is reduced. When the currentthrough switch 22 is decreased to the chopping current value at time t₃,then the voltage drop increases across now nonconducting switch 22. Thevoltage rise beyond the time t₃, in FIGS. 2 and 3, is controlled by thevalue of capacitors 30, 32 and 36, and the difference in slope betweenthe rising voltages is determined by the ratio of the capacitors 30 and32. Typically, the chopping current of a vacuum relay with refractorymetal contacts positioned as switch 22, is 10 amps in a switch capableof carrying 1000 amps. The charging of capacitor 32 through arcingswitch 22 must be sufficiently low that the current through switch 22can go below the chopping current level. When an acceptable voltage riseof 1 kv per microsecond is desired, capacitor 36 can be 10.sup.⁻⁶ faradsin value. To achieve a voltage division of approximately ten to one(i.e., the maximum voltage across switch 22 is ten times that across26), we require capacitor 32 to be ten times the size of capacitor 30.To insure that the charging current of capacitor 32 is less than thechopping current of switch 22, the size of capacitor 32 must be limitedin size to 10.sup.⁻⁸ F (in this example). Finally, the value ofcapacitor 30 must exceed the stray capacitance of switch 22 to beeffective. Conservative choices of capacitors 30 and 32 would be10.sup.⁻⁶⁹ F and 10.sup.⁻⁸ F, respectively. Thus, the voltage on thebreaker when current has stopped flowing, comprises 90 kv between bus 18and the intermediate bus at junction 24, and 10 kv between junction 24and bus 20. For this reason, the switch 26, the crossed field switchingdevice 28 and the larger capacitor 32 can be designed to hold offsmaller voltages than the maximum system voltages. With these valuesswitch 22 is a high voltage switch while switch 26 is a low voltageswitch.

Referring to FIG. 4, the circuit breaker 40 shown therein is connectedthe same as circuit breaker 10. It is connected to a power supply 42 andserially through a load 44 comprised of a resistor and an inductor.Circuit breaker 40 has buses 46 and 48 between which are seriallyconnected switches 50 and 52. Crossed field switch device 54 isconnected in parallel to switch 52 while serially connected capacitors56 and 58 are connected across buses 46 and 48, and in parallel toswitches 50 and 52. These components are the same as the components ofcircuit breaker 10, with the same examples of actual construction andratings. However, in order to prevent an offswitching operation to causetoo high a line voltage pulse, due to inductive or capacitivecharacteristics of the line, circuit breaker resistive branches 62 and64 are provided.

Circuit breaker branch 62 comprises a power resistor 66 connected to bus46 and to intermediate bus 68. Intermediate bus 68 is connected througha series connection of switch 70 and crossed field switch device 72.These switches are paralleled by a series connection of capacitors 74and 76, which are connected on their other end to bus 48. Theintermediate connection between the capacitors 74 and 76 is connectedbetween switches 70 and 72. Additionally, capacitor 78 is connectedbetween bus 68 and bus 48. Switches 70 and 72 correspond to switches 50and 54 in character, and are preferably of the same nature. Now, whenthe first section of circuit breaker 40 containing switches 50, 52 and54 is turned off, current is driven through power dissipating resistor66, closed switch 70 and conducting crossed field switch 72. When switch70 is opened and crossed field switch 72 is made non-conducting, thevoltage division thereacross is divided in accordance with the value ofcapacitors 74 and 76. These are chosen in the same ratio as capacitors56 and 58. Capacitor 78 again smooths voltage surges due tooffswitching.

Another branch 64 serves as an additional offswitching stage so thatfurther power can be absorbed in resistor 80 as offswitching proceeds.In this way, sequential offswitching results in power absorption in theresistors, together with reduction in line current. This example of thevoltage dividing DC circuit breaker illustrated in FIG. 4 shows themanner in which a sequential circuit breaker can employ the voltagedividing circuitry and equipment. Thus, it can be employed in asequential circuit breaker as illustrated in K. T. Lian U.S. Pat. No.3,534,226 or in a circuit breaker of the type described in K. T. Lianand W. F. Long U.S. Pat. No. 3,641,358. Similarly, it can be employed ina practical circuit breaker of the type described in the U.S. Pat. No.3,657,607 by W. Knauer. Thus, by employment of this circuit, voltagedividing can take place and the crossed field switch device need notcarry the full circuit voltage.

This invention having been described in its preferred embodiment, it isclear that it is susceptible to numerous modifications and embodimentswithin the ability of those skilled in the art and without the exerciseof the inventive faculty. Accordingly, the scope of this invention isdefined by the scope of the following claims.

What is claimed is:
 1. A voltage dividing DC circuit breaker forbreaking a circuit against line voltage and having circuit breakingcomponents of insufficient capability for withstanding open circuit linevoltage comprising:first and second buses in said circuit breaker; firstand second serially connected switches connected between said buses andconnected to an intermediate bus therebetween; an electronic.Iadd.crossed field .Iaddend.switch .Iadd.device .Iaddend.capable ofoffswitching all the current through said circuit breaker againstvoltage build up less than the open circuit voltage connected inparallel to said second switch between said intermediate and secondbuses; and first and second serially connected capacitors connected tosaid first and second buses and at their intermediate point connected tosaid intermediate bus, so that in normal conduction current flowsthrough said circuit breaker through said first and second switches inthe closed condition, and when said first and second switches areopened, said first switch arcs and said second switch forces the circuitcurrent through said electronic .Iadd.crossed field .Iaddend.switch.Iadd.device.Iaddend., and when said electronic .Iadd.crossed field.Iaddend.switch .Iadd.device .Iaddend.is offswitched, current throughsaid first switch decreases so that its arc extinguishes and currentbuilds up with the standoff voltage divided between said first bus, andsaid intermediate bus and said second bus in accordance with thecapacities of first and second capacitors. .[.2. The voltage dividing DCcircuit breaker of claim 1 wherein said electronic switch is a crossedfield switch device..].
 3. The voltage dividing DC circuit breaker ofclaim .[. 2.]. .Iadd.1 .Iaddend.wherein said second capacitor hasgreater capacity than .[.sad.]. .Iadd.said .Iaddend.first capacitor. 4.The voltage dividing DC circuit breaker of claim 1 further including aresistive circuit breaker branch comprising:a serial connection betweensaid first and second buses comprised of a first resistor, and a thirdswitch and a second electronic .Iadd.crossed field .Iaddend.switch.Iadd.device .Iaddend.and a series connection of third and fourthcapacitors connected in parallel to said third switch and said secondelectronic .Iadd.crossed field .Iaddend.switch device so that when saidfirst and second switches are opened and said first electronic.Iadd.crossed field .Iaddend.switch .Iadd.device .Iaddend.is turned off,current passes from said first bus to said second bus through said firstresistor, said third switch and said second electronic .Iadd.crossedfield .Iaddend.switch .Iadd.device .Iaddend.to absorb energy in saidfirst resistor and when said third switch is opened and said secondelectronic .Iadd.crossed field .Iaddend.switch .Iadd.device .Iaddend.isturned off, voltage thereacross is divided in accordance with thecapacity of said third and fourth capacitors. .[.5. The voltage dividingDC circuit breaker of claim 4 wherein said electronic switch is acrossed field switch device..].
 6. The voltage dividing DC circuitbreaker of claim 4 further including a second resistive breaker branchconnected between said first and second buses, said second resistivecircuit breaker branch having the same type of components as said firstcircuit breaker branch. .[.7. The voltage dividing DC circuit breaker ofclaim 6 wherein the electronic switch in said second resistive circuitbreaker branch is a crossed field switch device..].
 8. The method ofoffswitching DC current flowing from a first bus to a second bus andwhere first and second serially connected switches are connected betweenthe first and second buses and to an intermediate bus therebetween, withan electronic .Iadd.crossed field .Iaddend.switch .Iadd.device.Iaddend.connected between the intermediate bus and the second bus, andwith serially connected capacitors connected between the first andsecond buses and to the intermediate bus, comprising the stepsof:permitting conduction through closed first and second switches forcurrent flow from the first to the second bus; opening the second switchto force current flow through the .Iadd.crossed field .Iaddend.switch.Iadd.device .Iaddend.so that the second switch ceases conduction;opening the first switch so that it continues conduction of the circuitcurrent through arcing; turning off the electronic .Iadd.crossed field.Iaddend.switch .Iadd.device .Iaddend.so that the current through thefirst switch is reduced sufficiently low so that arcing of the firstswitch deionizes; and dividing the voltage rise across said first switchand said electronic .Iadd.crossed field .Iaddend.switch .Iadd.device.Iaddend.resulting from cessation of conduction through said firstswitch and said electronic .Iadd.crossed field .Iaddend.switch.Iadd.device .Iaddend.in accordance with the series capacitors connectedin parallel thereto.